Dimmer circuits are used to control the power provided to a load such as a light or electric motor from a power source such as mains power. Such circuits often use a technique referred to as phase controlled dimming. This allows power provided to the load to be controlled by varying the amount of time that a switch connecting the load to the power source is conducting during a given cycle.
For example, if voltage provided by the power source can be represented by a sine wave, then maximum power is provided to the load if the switch connecting the load to the power source is on at all times. In this way the, the total energy of the power source is transferred to the load. If the switch is turned off for a portion of each cycle (both positive and negative), then a proportional amount of the sine wave is effectively isolated from the load, thus reducing the average energy provided to the load. For example, if the switch is turned on and off half way through each cycle, then only half of the power will be transferred to the load. The overall effect will be, for example in the case of a light, a smooth dimming action resulting in the control of the luminosity of the light.
Modern dimming circuits generally operate in one of two ways—leading edge or trailing edge. In leading edge technology, the dimmer circuit “chops out” or blocks conduction of electricity by the load in the front part of each half cycle (hence the term “leading edge”). In trailing edge technology, the dimmer circuit “chops out “or blocks conduction of electricity by the load in the back part of each half cycle.
Since the load is connected to a high voltage or current source such as mains power, a defect in the circuit such as a short circuit, can lead to a sudden surge of high current, which can damage the load and any circuitry connected to the load. It is useful for the dimmer circuit to be able to detect the presence of such high, or overcurrent conditions, and act so as to remove the load and/or connected circuitry from the high current source.
The decision to act so as to remove the load and/or connected circuitry may be based upon the sensed current exceeding a preset threshold. A number of methods exist which provide a means and method of comparing the sensed current with a preset threshold.
In one method, the criteria for cutout is determined by the instantaneous current flowing through the dimmer exceeding a pre-determined threshold level, particularly for the condition when a power device is commencing conduction using controlled transition time while the instantaneous line voltage is high, in that the instantaneous power dissipation imposed upon the power device while a short-circuit load condition exists, is high.
In another existing method, the criteria for cutout is determined by the product of the instantaneous voltage appearing across the dimmer and the instantaneous current flowing through the dimmer exceeding a pre-determined threshold level i.e. instantaneous power level in the power semiconductor, however, such circuit designs are complex and expensive to design and manufacture.